A new effort concentrating on animal models of AIDS was undertaken in August, 1987. Two model systems are being established: 1) a primate model using the simian immunodeficiency virus (SIV), and, 2) a feline model using the feline T-lymphotorophic virus (FTLV). both of these viruses belong to the lentivirus subfamily and induce chronic persistent infection and immunodeficiency in their respective hosts. SIV is genetically closely related to the human AIDS viruses, while FTLV appears to be much less related to the human or primate viruses based on a lack of antigenic cross reactivity (sequence data is not available). Thus, these two model systems will allow an in-depth examination (in vitro and in vivo) of the molecular biology, pathogenesis, and host immune responses, and will afford the opportunity to evaluate systematically candidate vaccine approaches to immunoprophylaxis and antiviral therapies.